UK border agents found 68 people, including 15 children, locked in containers aboard four trucks that had arrived from the Netherlands at a port in Essex, CNN quoted UK authorities as saying on Friday.
Those found are 35 Afghans, 22 Chinese, 10 Vietnamese and one Russian, the report said.
The children's nationalities have not been disclosed. There are two pregnant women.
According to a spokesman of the UK Home Office, the discovery was made when the trucks were searched at Harwich International Port, northeast of London, late Thursday.
All 68 are now in the hands of the UK Border Force, which is part of the Home Office.
Four Polish nationals who were driving the trucks "have been arrested on suspicion of facilitating illegal immigration" and are undergoing questioning by the Home Office's criminal investigations team, the spokesman said.
Deputy Prime Minister Nguyen Xuan Phuc last month ordered an investigation following media reports that 3,000 Vietnamese children had been illegally trafficked into the UK and kept as slaves in cannabis factories, brothels and private homes.
Findings will have to be reported to Prime Minister Nguyen Tan Dung this month, Phuc said.
In May 23, an exposé on the Guardian said that around 3,000 Vietnamese children are in forced labor in the UK, used for financial gain by criminal gangs running cannabis factories, nail bars, garment factories and brothels.